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International Journal of Engineering Trends and Technology (IJETT) – Volume 1 2 N umber 7 - Jun 2014 ISSN: 2231 - 5381 http://www.ijettjournal.org Page 331 Wireless Power Transmission f or Charging Mobiles Aakib J. Sayyad 1 , N. P. Sarvade 2 1 PG Student, 2 Associate professor , 1,2 Electrical Engineering Department, VJTI, Mumbai, India Abstract — Wireless power transmission holds a promise able future for generating a smal l amount of electrical power f o r charge mobile wirelessly. growing Importance in wireless field which has numerous benefits such as it would completely eliminate the need of carrying charger along with mobiles, no more need of keeping mobiles nearer to socket as cables have shorter length than ar ea covered by wireless field. This paper present s architecture of wireless transmission for mobile systems(WPTM) , design specifi cations with simulation of quarter wave printed monopole antenna which i s best suitable for the project because of its high gain & efficiency and low loss. A S chottky diode based charge pump circuit performance is also studied with simulations result with input of 0dBm and - 10dBm . Purpose of this paper is to build an architecture for wireless low powertransmission system which is , simp le , affordable robust and to identify promising directions for future research. Keywords — W ireless power transfer for mobile(WPTM),Nikola Tesla, λ/4 printed monopole antenna, schottky diode . I. I NTRODUCTION The technology and theory behind wireless charging exist around for a long time – the idea was initially suggested by Nikola Tesla, who demonstrated the principle of wireless charging at the turn of the century [3] . This paper implies wireless transmission of power using RF waves. A suitable charging circuitry is demonstrated which can capture high frequency signal and convert it into DC signal. The application scenario considered in this paper is as follows. A low noise AC signal at 915MHz is to be generated using a signal generator . At receiver end a  /4 printed monopole antenna tu ned at same frequency will capture signal transmitted, output of which is to be given to charge pump circuit which is multi stage voltage multiplier . Finally output is to be measured at output of voltage doubler. This paper is organized as follows: Section II gives a brief description of background of wires power transfer system and information about basic antenna and mobile charger mechanism . Specifications of monopole antenna are proposed in s ection III . SectionIV deals with charge pump circuit ry . Section V shows the simulation results, Section V I represents the conclusion of the study. Finally future work will be referred to in section VII. II. W IRELESS P OWER T RANSFER Nikola Tesla is the one who first proposed the idea Wireless Power Transmission and demonstrated “the transmission of electrical energy without wires" in 1891[2]. In 1893, heinvented the illumination of vacuum bulbs without using wires for power transmission at the Wor ld Columbian Exposition in Chicago. The Ward enclyffe tower was designed and constructed by Tesla mainly for wireless transmission of electrical p ower [3] . Energy harvesting has been in the focus of the research community in recent years. There are numerous sources of power that energy harvesting can benefit from, and solar energy harvesting is one of the key examples since it has the highest energy density among other candidates. However, it has a drawback of being able to operate only wh en sunlight is pres ent. In [11 ], a solar energy harvesting module is used to power a sensor mote. Vibrational energ y harvesting is presented in [12 ] while harvesting energy from thermoelectric device attached to human is discussed in [13 ]. Small amount of work has been done on RF energy harvesting due to its low energy density. Wireless battery charging system using RF energy harvesting is discussed in [7]. In [5 ], the energy of 60µW is harvested from TV towers, 4.1 km away, and is able to operate small electronic device. Ambie nt RF energy harvesting with two systems has been studied in [14 ]. Recently prototypes for such RF harvesters have b een developed in the academia [4], [8], [9 ], as well as commercial products have been introdu ced by the industry [15 ]. A. Wireless power transm ission system William C. Brown has designed, developed a unit and demonstrated to show how power can be transferred through free space by microwaves [6] . In the transmission side, the microwave power source generates microwave power and the output power is controlled by electronic control circuits. In the receiving side, a antenna receives the transmitted power and converts the microwave power into DC pow er. International Journal of Engineering Trends and Technology (IJETT) – Volume 1 2 N umber 7 - Jun 2014 ISSN: 2231 - 5381 http://www.ijettjournal.org Page 332 B. Architecture of wireless power transmission for mobile( WPTM ) systems Fig. 1 shows the architecture of the Wireless Power Transmissions for mobile systems. Signal generator transmits a low noise AC signal at 915MHz into space while the antenna of the r eceiver captures it to convert it into electricity. The radio waves cannot be captured efficiently if the length of an antenna is not accurately right for the frequency used . It should not be too long, or too short. A good transmitting antenna is always a good receiving antenna, and for ordinary uses, one antenna can be served for both functions. Output of receiving antenna is to be connected to charging circuitry which converts high frequency AC signal to DC, to get more of output voltage doubler circui t is to be connected which rectifies as well as doubles the voltage present at input, number of stages used is directly proportional to number of times voltage doubles. Finally required output is to be taken at multi stage voltage doubler’s end. Fig. 1 . Architecture of WPT M System C . Scope and discussion WPT M system λ /4 printed monopole antenna is proposed which very simpler in structure, easy to construct and have low cost. In this paper specifications of monopole antenna are discussed. A multistage voltage multiplier which is used as a charging circuit is proposed. Voltage multiplier has an advantage of doubling the input voltage, though the output is not exactly DC. It is an AC signal with a DC offset. III. Q UARTER W AVE P RINTED M ONOPOLE A NTENNA FOR AND D ISCUSSION 868/915 M HZ Dipole and monopole ar e similar in characteristics as well as structure. The difference being that a monopole has one connec tion point to the circuit, whereas a dipole has two connection points. For this p roject, the monopole antenna is the antenna of choice because it is simple in design and simple in structure . It consists of a copper wire p iece with one end connected to the circuit, whereas the other left open [16] . Good quality of this antenna is that it has quiet large operating frequency range . For research, it is helpful because exact tuning of the antenna is not required. A . Basic pro per ties of a quarter wave monopole antenna Q uarter wave monopole is a ground plane dependent antenna which must be fed single - ended. The antenna must have a ground plane to be quiet efficient, and ideally the ground plane should spread out to at least a quarter wavelength, or more, around the feed - point. The size of the ground plane affects the gain, resonance frequency and impedance of the antenna. The length of the monopole PCB trace mainly determined by the resonant frequency of the antenna , but because of having very wide gain bandwidth of a quarter wave monopole, the antenna length is not that critical. But like any other antenna types, the gain of a quarter wave monopole will vary if parameters, such as case/box materials, distance to the ground plane and size of the ground plane, width and thickness of the PCB trace are varied. If any of these parameters are changed , it will affect the gain of antenna . B . Determining the length of the printed monopole antenna For an application operating i n the 902 - 928MHz band , the antennacan be tuned t o a frequency in between the band . Because of the fairly wide gainbandwidth of the antenna, the antenna will show good overall performance in the wholefrequency range 868 - 928MHz when tuned to 900MHz. The antenna is to be fabricated on a standard 1.6mm FR4 substrate material with dielectricconstant e r of 4.4 at 900MHz.The width of the monopole trace is selected to W = 1.5mm. The wavelength in free air is λ 0 = 333mm. So the guided wavelength λ g on the FR 4 substrate is approximated to about  g ˜ 0.75 ·  0 = 0.75 · 333mm ˜ 250mm The approximate, physicallength of a printed quarterwave monopole antenna will be then L = 250/4 = 62.5mm P rovided that , the size of the available ground plane should be close to the ideal and that the antenna trace should be uniformly surrounded by the FR4 substrate.When implementing the monopole as a trace on the PCB, the length of the trace should beextended somewhat to allow for some fine - tuning of the antenna to resonance at 91 5 MHz. Ifthe size of ground plane is approaching the ideal size and the antenna trace isuniformly surrounded by the FR4 substrat e, then the length of the trace should be extended by about 20% [15 ]. C . RF energy specifications The main challenge faced in harv esting RF energy is the freespace path loss of the transmitted signal with distance. The Friis transmission eq uation relates the A low noise AC signal at 915MHz transmitted from signal generator Rectifying or charging circuitry converts high frequency AC signal into DC DC power given to Mobile batteries Antenna at the receivi ng end catches it and converts it into electricity International Journal of Engineering Trends and Technology (IJETT) – Volume 1 2 N umber 7 - Jun 2014 ISSN: 2231 - 5381 http://www.ijettjournal.org Page 333 received (Pr) and transmitted (Pt ) powers with the distance R as, (1) W here Gt and Gr are antenna gain , and λ is the wavelength of the transmitted signal. The received signal strength, diminishes with the square of the distance, requires special sensitivity considerations in the circuit design. Moreover, FCC regulations limit the maximum transmission power in s pecific frequency bands. For example, in the 900 - MHz band, this maximum threshold is 4W[11]. IV. C HARGING C IRCUITRY It is a cir cuit that when given an AC input, it is able to output a DC voltage typically larg er than a simple rectifier generate s . It is si mply an AC to DC converter that rectifies the AC signal and clamps the DC level. This circuit is used in many electronic devices today. The circuitis much mo re complex than the charge pump used in this work . Power converter s have a l ot of protective circuitry in addition with circuit to reduce noise. In fact, it is for safety regulation .A ny power - conversion circuits use a transformer to is olate the input from the output,which prevents overload of the circuit and user injury . For work of low power ener gy harvesting , a circuit of mentioned complex would require more power than available, so that it would be very inefficient and may not function. Which causes need to use a simple de sign. As mentioned due to an advantage of getting more voltage at output t han the input voltage multiplie r circuit is chosen to be used. A. Voltage multipliers Given circuit is called a voltage multiplie r because according to theory, the voltage that is received on the output is almost twice that at the i nput if only one stage i s considered. The schematic is shown in figure 2which represents one stage of the circuit. The input is rectified by diode D2 and capacitor C2 in the positive half of the cycle, and in negative cycle by diode D1 and capacitor C1 . D uring the next positive half - cy cle, the voltage stored across capacitor C1 during the negative half - cycle is transferred to capacitor C2. Thus, the voltage on capacitor C2 will be roughly two times the peak voltage. The most interesting feature of this circuit is that b y adding more stages and connecting them in series , we can get more voltage at the output. Output is not exactly DC. It is essentially an AC signal with a DC offset. This can be seen in Figure 3. If a second stage is added in series of the first, output voltage will clamp to more value . Fig. 2. Votage Double Schematic Fig. 3 . Voltage multiplier Waveform Therefore, the more the stages added, more will be the output volt age irrespective of the input. If open circuit output voltage is Vo and internal resistance is Ro . and n circuits are put in series connected to a load of R L , the output voltage is given by , (2) B . System Specifications There are many variables in the system that can cause change in the voltage developed. The stage capacitors , the number of stages needs to be optimized . Also, a value of capacitor used at the output which acts as a filter to provide a f lat DC signal and store charge , needs to be determine d. There are no fixed values for any of these parameters . The only specified value is the operating frequency of the system . This frequency is chosen to be 915MHz. 1) Choice of Diodes : One of the crucial requirements for the energy harvesting circuit is to be able to operate wi th weak i nput RF power. For a typical 50 antenna, the - 20 dBm re ceived RF signal power means amplitude of 32 mW. As the peak voltage of the ac signal obtained at the antenna is generally much smaller than the diode threshold [10 ], diodes with lowest possible turn on voltage are preferable. Moreover, since the energy harvesting circuit is operating in high frequencies, diodes with a very fast switching time need to be used. Schottky diodes use a metal – semiconductor junction instea d of a semiconductor – semiconductor junction. This allows the junction to operate much faster, and gives a forward voltage drop of as low as 0.15 V. Diode from Agilent technologies HSMS - 2852 is to be employed, which has turn on voltage of 150mv. International Journal of Engineering Trends and Technology (IJETT) – Volume 1 2 N umber 7 - Jun 2014 ISSN: 2231 - 5381 http://www.ijettjournal.org Page 334 2 ) Number of stages: The number of stages used in charging circuitry , has the largest effect on the output voltage. Generally, the voltage of the output increases as the number of stages increases. However, practically force a limit on the number of permissible stage s, and in turn, the output voltage. Actually , the voltage gain decreases as number of stages increases due to parasitic effect of the capacitors of each stage, and finally it becomes negligible. 3 ) Stage capacitance: The stage capacitance is very sensitive parameter of the system . Sometimes, minimal changing can cause drastic effect on output voltage . The cap acitance parameter is given by the following equation. Q=C.V(t) In above Equation, the voltage across capacitor is inversely proportional to t he capacit ance in relation with the charge . To hold same charge value of capacitance must be minimum to have more voltage across the capacitors. 3 ) Output capacitance: Output capacitance affects least on the overall system . It only affects the s peed of the transient response. B igger the value for the output capacitance, slower will be the voltage rise time. This does n ot mean that no capacitor should be used. Without a capacitor there, the output will be a DC with ripple which is unfaithful. V. S IMULATION Design an d simulation of antenna is done in sonnet software whereas, Simulation of circuit is to be done in Agilent Advanced Design System(ADS) software. The harmonicbalanced analysis (a frequency domain method) is proposed in this work since objective of the project i s to compute the steady state solution ofa nonlinear circuit. The alternate method, the so called transientanalysis that is undertaken in the time domain is not usedowing to the reason that it must collect sufficient samples for thehighest frequency compone nt. This involves significant memoryand processing requirements. Depending on the results of simulation, number of stages is to be selected. A .Antenna As said, Antenna was designed in ‘sonnet’ software and simulation results were studied. Design consider ations were as discussed in 3.3. Figure 5 shows the design of λ /4 printed monopole antenna which is about 100mm in length and 10mm in width. Lossless material is chosen to analyse results. Fig. 4 . Design of λ /4 printed monopole antenna The S11 parameter which is relative loss and Far field respo n se of Gain with respect to frequency were as shown in figure 6 and figure 7 respectively. Fig. 5 . S11 parameter in d B with respect to Frequency in MHz Also the gain at 915MHz is 6.3dBi which is actually more than desirable, however use of lossy material will decrese it’s gain . B. Voltage multipliers The energy harvesting circuit which is voltage multiplier is simulated using Agilent Advanced Design System (ADS) software. Snapshot of the circuit is given in Figure 7 . Simulation results were analysed with respect to number of stages and value of stage and output capacitance. Output from receiving antenna is assumed to be in between - 10dBm to 0dBm. So the output of circuit is simulated with respect to two different inputs, i.e. at - 10dBm and at 0dBm. It is found that stage capacitor of 1nF gives best optimized result, if capacitance lowered further, output voltag e decreases. Output capacitance has less effect on value of output voltage. B igger the value for the output capacitance, the slower will be the voltage rise time. Lowering capacitance to pF lowers current drastically so capacitance value is chosen to be 1nF. International Journal of Engineering Trends and Technology (IJETT) – Volume 1 2 N umber 7 - Jun 2014 ISSN: 2231 - 5381 http://www.ijettjournal.org Page 335 Fig. 6. 12 stage Voltage multiplier in ADS Sim ulation results for stage 1 to stage 12 were studied. T he voltage of the output increases with increment in number of stages, however the voltage gain decreases as number of stages increases due to parasitic effect of the constituent capacitors of each sta ge, and finally it becomes negligible. Table 1 and Table 2 shows the simul ation results with respect to 0 dBm and - 10dBm input respectively. TABLE I O/P V OLTAGE AND O/P C URRENT T AKEN AT I NPUT OF 0 DBM Number Of Stages Output Voltage( Vout) Output Current(Iout) 1 1.04V 590uA 2 2.02V 1.2mA 3 2.92V 1.7mA 4 3.74V 2.2mA 5 4.48V 2.7mA 6 5.12V 3.1mA 7 5.66V 3.5mA 8 6.10V 3.9mA 9 6.44V 4.0mA 10 6.69V 4.1mA 11 6.88V 4.3mA 12 6.91V 4.4mA VI. C ONCLUSION AND D ISCUSSION W ireless low power transmission system is studied. Among all the antennas of λ /4 printed monopole antenna is found to be suitable for low power transmission such as mobile charger. Charging circuit model is proposed . The concept of charging a cellular phone battery using wireless RF energy harvesting can be feasible. While voltage across output is desirable but output current is quiet lower showing that battery or mobile phone will require much time to charge. Wireless low power transmission system would completely e liminate carrying chargers for mobile phones or tabloids. VII. F UTURE W ORK The future studies mainly concentrate on reducing physical size of antenna and embed it in mobile itself t o give high efficiency and less loss . Also designing high level power transmission system for charging laptops have good scope of work. TABLE II I O/ P V OLTAGE AND O/ P C URRENT T AKEN AT I NPUT OF - 10 DBM R EFERENCES [1] http://cleantechindia.wordpress.com/2008/07/16/indiaselectri city - transmission - and - distribution - losses/ [2] Nikola Tesla, My Inventions, Ben Joh nston, Ed., Austin, HartBrothers, p. 91,1982. [3] Nikola Tesla, “The Transmission of Electrical Energy Without Wires as a Means for Furthering Peace,” Electrical World and Engineer. Jan. 7, p. 21, 1905. [4] PrusayonNintanavongsa, UfukMuncuk, Kaushik Roy Chowdhury “Design Optimization and Implementation for RF Energy Harvesting Circuits” IEEE JOURNAL ON EMERGING AND SELECTED TOPICS IN CIRCUITS AND SYSTEMS, VOL. 2, NO. 1, MARCH 2012 pg no 24 - 33. [5] W.C. Brown, J.R. Mims and N.I. 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Van Hoof, “Thermoelectric converters of human warmth for self - powered wireless sensor nodes,” IEEE Sensors J. , vol. 7, no. 5, pp. 650 – 657, May 2007. [14] D. Bouchouicha, F. Dupont, M. Latrach, and L. Ventura, “Ambie nt RF energy harvesting,” in IEEE Int. Conf. Renewable Energies PowerQuality (ICREPQ’10) , Mar. 2010, pp. 486 – 495. [15] Nordic s Semiconductor,[online]. Available: https://www.nordicsemi.com/eng/nordic/download_resource/ 8066/3/45367956.